Aqueous slurries, which basically comprise an aqueous medium and particulates, are commonly used in the oil and gas industry to transport particulates through a pipe or tube, either on ground, or from the surface to a subterranean formation or from a subterranean formation to the surface. The most commonly used particulates include sand, ceramic particulates, glass spheres, bauxite (aluminum oxide) particulates, resin coated particulates and synthetic particulates. The particulates usually range in size from about 10 to about 100 U.S. mesh, i.e., about 150 to 2000 μm in diameter and normally have significantly higher density than water. For example, the density of sand is typically about 2.6 g/cm3 while the density of water is 1 g/cm3. Aqueous slurries are widely used in petroleum industry, which include hydraulic fracturing and drilling operations. To make a relatively stable slurry, the particulates must be suspended in a liquid medium for a lengthy period of time at static and/or dynamic conditions, and therefore the viscosity or viscoelasticity of the fluid must be sufficiently high in order to be able to suspend particulates. The most commonly used method for increasing viscosity or viscoelasticity of an aqueous liquid is by adding a viscosifier (for example, a natural or synthetic polymer) or a viscoelastic surfactant to the liquid.
Hydraulic fracturing is a technology commonly used in the petroleum industry to enhance oil and gas production from a subterranean formation. During the operation, a fracturing fluid is injected through a wellbore into a subterranean formation at a pressure sufficient to initiate fractures in the formation. Frequently, the fracturing fluid comprises particulates, commonly known as proppants, suspended in the fluid and transported as a slurry into the fractures. For example, following the initiation of the fractures the slurry transports the particulates into the fractures. At the last stage of the fracturing operation, fracturing fluid is flowed back to the surface leaving proppants in the fractures forming proppant packs to prevent fractures from closing after pressure is released (i.e., the particulates “prop” open the fractures). The proppant packs provide highly conductive channels that allow the hydrocarbons (e.g., oil and/or gas) to seep through the formation to the wellbore more efficiently. Proppants, including sands, ceramic particulates, bauxite particulates, glass spheres, resin coated sands, synthetic particulates and the like, are known in the industry. Among them sands are by far the most commonly used proppants. As noted above, the proppants normally range in size from about 10 to 100 U.S. mesh, which is about 150 to 2000 μm in diameter.
Fracturing fluids in common use include various aqueous-based and non-aqueous based (i.e., hydrocarbon-based) fluids. Due to their low cost and high versatility, aqueous-based fluids are preferred and most commonly used. To better transport particulates, water-soluble viscosifiers, such as polymers (i.e., linear or cross-linked polymers) or viscoelastic surfactants are added to increase fluid viscosity. For example, a polymer, such as guar gum or its derivatives, is added into an aqueous liquid wherein the physical entanglement of polymer chains increases the fluid viscosity and thus its suspension capability. To further enhance fluid viscosity, it is common to chemically cross-link polymer chains by certain chemical compounds forming chemically cross-linked gel. Guar gum cross-linked by borates is one example of this. Compared to a cross-linked fluid, linear gels, i.e., fluids containing sufficient amount of polymers without cross-linking, cause less formation damage and are more cost-effective, but have relatively poor suspension capability. In recent years, slick water, i.e., water containing very small amount of friction reducing agent, which normally ranges from 0.015% to 0.1%, preferably 0.02% to 0.06%, of the fluid, has been widely used as a fracturing fluid, especially for fracturing shale or tight formations. Various water-soluble polymers, including guar gum and its derivatives as well as polyacrylamide and its derivatives, have been used as friction reducing agents. Polyacrylamide copolymers, which contain other monomers in addition to acrylamide monomers, are commonly used as friction reducing agents in hydraulic fracturing operations. One such type of copolymer is a hydrophobically modified polyacrylamide copolymer.
Viscoelastic fluids are the fluids that exhibit both viscous and elastic characteristics when being subjected to stress and are widely used to make aqueous slurries to transport particulates. Basically, the viscosity of the fluid works to slow down the rate of particulate settling out of suspension, while the elasticity helps to suspend the particulates. Under dynamic conditions, agitation or turbulence further help stabilize the slurry. Therefore, conventional methods of making stable particulate slurries focus on manipulating the rheological properties of the fluid by adding sufficient amounts of a viscosifier, such as a water-soluble polymer, to the slurry. It is not unusual that a polymer is used together with a foaming agent to improve the rheology and reduce the cost.
As noted above, the last stage of a fracturing treatment involves the flowing of the fracturing fluid back to the surface while the proppants are left in the fractures. However, it is not unusual for a significant amount of proppant to be carried out of the fractures and into the wellbore along with the fluids being flowed back out of the well. This process is known as “proppant flowback”. Proppant flowback after fracturing treatments has long plagued the petroleum industry. It is highly undesirable because it not only reduces the amount of proppants remaining in the fractures (thus, leading to reduced fracture conductivity), but also causes significant operational difficulties. U.S. Pat. No. 6,047,772 indicates that different methods have been tried to address the problem of proppant flowback. In one method, resins are used to coat the proppant grains to make them tacky so that they stick together to reduce proppant flowback. This method is expensive, and operationally challenging.
There still exists a need for compositions and methods of making slurries that will form a stable proppant pack in the fracture formations and resist flowing back to the surface, while at the same time being cost-effective and operationally simple.
When drilling subterranean formations for oil and gas, aqueous-based drilling fluids are normally used. During the drilling process large amounts of particulates, called cuttings, are generated. Cuttings have different sizes ranging from fines to pebbles. The drilling fluid is circulated through the wellbore to make a slurry with the cuttings in situ and subsequently transport them out of wellbore. In most cases, polymers as well as clays are added to the drilling fluids to increase their viscosity/viscoelasticity in order to transport the cuttings efficiently. However, polymers and clay fines can easily penetrate into pores or thin fractures in the formation and significantly reduce formation permeability, especially at near wellbore. Reduced formation permeability impedes oil and/or gas production. Therefore it is highly desirable to have a drilling fluid that can make stable slurry in situ with the cuttings and transport them out of the wellbore, while at the same time cause less formation damage (i.e., a fluid that does not impede the permeability of the formation).
In oil sand operation massive amount of sands are left after oil is stripped from the sand surface. Finding a more cost efficient way to transport sands efficiently over distance through pipelines has long been required in the industry.
U.S. Pat. Nos. 7,723,274 and 8,105,986 disclose a different way of enhancing particulate transportation using a slurry. Unlike the conventional way, which focuses on improving fluid rheology (as discussed above), these patents teach that by rendering the particulate surfaces sufficiently hydrophobic, gas bubbles become attached to the particulate surfaces, thus buoying the particulates, and consequently resulting in the formation of stable slurry without requiring viscosifying of the fluid. Moreover, the spontaneous attachment of bubbles to different particulates bridges the particulates together resulting in particulate agglomeration (aggregation). This is also known as gas bridging in the scientific literature. The slurry can be used to effectively transport particulates in different applications, particularly in hydraulic fracturing operation.